This invention relates to conversion of component video signals of the YUV format of a television system to RGB (Red-Green-Blue) signals of a computer system. In particular, this invention relates to conversion of digitized YUV signals to RGB signals for use as part of a computer displayable image.
Methods and apparatus are known for processing composite television signals through an analog decoder to produce separate analog Red, Green, and Blue (RGB) signals for projection of an image in a display device. It is also known to provide television image processing systems having capture or frame grabbing capabilities in connection with merged display images.
The traditional approach to reproduction on a computer display has been to convert the individual analog Red, Green and Blue signals obtained from analog conversion of composite television signals into separate digital signals and then to provide the digital signals to appropriate display reproduction circuitry of a computer.
Color reproduction requires a color space defined by a minimum of three signals. The composite color television signal can be constructed of three component signals, Y, U and V. The Y signal represents to luminance or picture brightness. The U signal is the scaled difference between the blue value and the luminance (B-Y). The V signal is the scaled difference between the Red value and the luminance (R-Y).
Computer displays of color, however, cannot be reproduced directly from the YUV signals on an RGB-type display device, and hence color space conversion to the RGB format is necessary.
Prior art color space conversion has involved for example a digital 3 by 3 matrix multiplication typically using at least 8-bit multipliers, whereby any arbitrary three-component color system can be converted to any other three-component color system. The approach, although flexible, is expensive or is slowed by direct computation of values.
A second technique for conversion involves bifurcating the multiplication process by first applying a coefficient to a component through a table lookup technique according to a predefined transformation and then summing three components to produce results. While this approach is relatively inexpensive, it lacks the flexibility of a multiplication transformation.
A possible approach is to provide a lookup table having the equivalent of three sets of data inputs (24 lines) and one set of outputs (8 lines), which would be expected to require a lookup table having a capacity of 16 Megabytes. At present, the largest commercial practical lookup table device has a capacity of several orders of magnitude less than such a table. What is needed therefore is a converter having the flexibility of a true matrix multiplier at an expense and level of efficiency exceeding implementation of straightforward table lookup structures.
A sample of the patents in this general field includes U.S. Pat. No. 4,823,120 for an enhanced video graphics controller. This patent describes a current product of Apple Computer with which the output signal of a product incorporating the present invention might be used.
U.S. Pat. No. 4,739,313 describes a device for converting composite video to RGBI signals in which the resolution is limited to only 16 colors. The range of choice of colors is considerably less than the present invention and would be considered inadequate for a digital reproduction of a color television image.
U.S. Pat. No. 4,720,745 describes a method and apparatus for enhancing NTSC composite video signals as part of a conversion into RGB components. This invention does not deal with color conversion but is basically a spatial filter to minimize noise.